blocking at truss heels

[AlpineEngineer]

I have looked at some former discussions on blocking at truss heels and couldn't find an answer to my question. I am having a disagreement with a building inspector over this and wanted to run it by you guys. I have specified blocking between the truss heels over my shear walls to transfer the shear forces from the roof diaphgram to the shear wall, I have detailing for roof sheathing nailing into the blocks and clips to connect the blocks to the top plates. I don't see why blocking would be required elsewhere.

The inspector wants blocking everywhere. I do see in the 2003IRC R802.8 that lateral support is required where the depth to thickness ratio of the truss/rafter exceeds 6:1, which we do exceed due to high heels on the trusses. But the H2.5 hurricane clips provide, in my mind, some degree of lateral support (150#). Furthermore the inspector indicates that the roof sheathing doesn't need to be nailed into the rest of this blocking; thus it wont do anything to for shear transfer. I guess all it will do is prevent truss rollover? It seems to me the H2.5 clips are enough to prevent rollover. If all these blocks do is prevent rollover do they need to be full hieght? Curious what you guys specify.
Thanks guys.

 

[tngolfer]

Obviously I do not know the inspectors reasoning but here is a possible train of thought.

Not sure about IRC, but IBC refers to the NDS code. In the NDS code, section 4.4.1 refers to the stability of bending members. For a 6 < d/b < 7, "... ends at points of bearing shall be held in position to prevent rotation and/or lateral displacement."

I'm assuming the truss is constructed of solid sawn lumber and the inspector feels it should be subject to these specs.

 

[DaveAtkins]

If you only put blocking over the shear walls, you must be sure the roof sheathing has adequate capacity to resist diaphragm force over just the length of these shear walls. You can't use the entire diaphragm, because without blocking no shear transfer can take place.

For stability at high truss heels, I usually see the exterior wall sheathing run up the heels and get nailed to the heels.

DaveAtkins

 

[msquared48]

Remember here that the double top plate is a drag strut / shear collector from the roof diaphragm. Copnsequently, as Dave said, the blocking must be in place between the trusses all around the limits of the diaphragm to allow the shear force to gradually transfer from the roof diaphragm to the double top plate. Otherwise you ruin into other problems.

Yes, rollover is one reason for the blocking, while another is to allow space for roof venting (with screened vent holes), and to prevent the intrusion of unwanted guests, such as birds, squirrels, rats, mice and raccoons (that's why they call it bird blocking!).

Mike McCann
MMC Engineering